1. Field of the Invention
The present invention relates to a projection display technology, and more particularly, to a high efficiency liquid crystal display projection system.
2. Description of Related Art
The projection liquid crystal display technology has been a usual technology. The traditional liquid crystal display projection system mainly uses the reflective liquid crystal on silicon (LCOS) panel to process the colors and the gray levels of the image pixels. One of the main characteristics of the so-called reflective LCOS panel is that most of the driving devices are formed on the lower substrate while the liquid crystal layer is formed between the upper and the lower substrates. The light source enters the lower substrate from the upper substrate and the light is then reflected from the reflective layer of the lower substrate. Therefore, the reflected light will not be blocked by the driving devices and the utility efficiency of lights can be improved.
FIG. 1 shows a traditional liquid crystal display system. In FIG. 11, a light source 100 emits a white beam 102. The white beam 102 enters a dichroic mirror 104 to be split into a blue beam 108 and a red/green (R/G) mixing beam 106. The R/G mixing beam 106 is then incident to another dichroic mirror to be split into a red beam 116 and a green beam 118. The light path and the mechanism of the blue beam 108 is first described. The non-polarized blue beam 108 comprises P-polarization and S-polarization. Then, the blue beam 108 enters a polarized beam splitter (PBS) device 110a. The functions of the PBS device include reflecting S-polarized light but allowing P-polarized light to penetrate through. Accordingly, the PBS device 110a will reflect the S-polarized light of the blue beam 108, which then enters the reflective LCOS panel 112a. The reflective LCOS panel 112a contains a pixel region. Through controlling the liquid crystal molecule rotation of the corresponding pixels, the S-polarized blue light will tilt to produce a new polarization state, comprising partial S-polarization and partial P-polarization. The amount of P-polarization varies according to the desired gray level, generating a gray level of colors in cooperation with the PBS device 110a. 
The blue light that is reflected back to the PBS device 110a by the reflective LCOS panel 112a contains P-polarization based on the requirement of the image pixel. This P-polarized blue light can penetrate through the PBS device 110a to be incident to a color-combination prism 120. The amount of P-polarization is determined by the blue light gray level required by the image. If blue light is not required, the value of the P-polarization will be zero. Hence, no blue light will penetrate through the PBS device 110a. As a result, the value of the P-polarization increases when the blue light gray level increases.
Based on the same mechanism, the red beam 116 is reflected by a reflective mirror and enters the PBS device 110b and then gets reflected to the PBS device 110b by the LCOS panel 112b, wherein the P-polarized red light will enter the color-combination prism 120.
Similarly, the green beam 118 is reflected by a reflective mirror and enters a PBS device 110c and then gets reflected to the PBS device 110c by the LCOS panel 112c, wherein the P-polarized green light will enter the color-combination prism 120.
The color-combination prism 120 receives the image lights of three colors to form an image 122. This image 122 can be projected to a screen. This type of liquid crystal display projection system processes the three primary colors, (red/green/blue, R/G/B), respectively, hence, it is bigger in volume with a higher manufacturing cost and a poorer utility efficiency of lights.
FIG. 2 shows a traditional dual-panel liquid crystal display projection system. In FIG. 2, the light source 200 of R/G/B lights emits light through a PBS device 202 in succession. Since human eyes experience a phenomenon known as visual retention, therefore, when the light emitted by the light source 200 of R/G/B lights enters the human eyes within the range of visual retention, the overlap of R/G/B lights results in what is perceived as colors by the human eyes.
As a result, the projection system shown in FIG. 2 requires only one PBS device 202, but two LCOS panels, namely 204a and 204b. For instance, after the light source 200 of R/G/B emits lights through the PBS device 202, the P-polarized red light 206 will penetrate through the PBS device 202 to be reflected by the LCOS panel 204b and the polarization varies according to the requirement of the gray level, which might be converted to S-polarization. Subsequently, the reflected PBS device 202 will reflect out a red beam 210. The generation mechanisms for green light and blue light are the same as the aforementioned, which will not be described again. In addition, the PBS device 202 also reflects a S-polarized red light 208, which enters the LCOS panel 204a to be converted to a P-polarized red light 220. This P-polarized red light 220 and the S-polarized red light 210 form one red light image. Since there are two LCOS panels, namely 204, the utility efficiency of lights is increased. Moreover, only one PBS device, namely 202, is needed because the light source of R/G/B lights emits light in succession.
Furthermore, the light-emitting surface of the traditionally used light source gives off uneven brightness. Thus, the choice of the light source affects the illumination of display.
Although different designs of liquid crystal projection system have been developed based on the traditional technology, there is still room for further research and development.